The Earth in Space
This view of Earth as a blue marble, first seen by Apollo astronauts in 1968, captures the unique beauty of our home planet. The only world known to support life, this fragile sphere hovers in the blackness of space in stark contrast to its lifeless moon beneath. Every ocean ever sailed, every mountain ever climbed, every life ever lived could be covered by the outstretched hand of the astronaut taking the picture.
Apollo 8 view of Earth
Our solar system is located in a benign region of the Milky Way galaxy, such that Earth is not bombarded with excessive high energy radiation from stellar neighbors. Within our solar system, Earth is positioned at just the right location with respect to Sol (our sun) to enable water to exist in all three phases. Sol itself is a stable, middle-aged star with no recent history or inclination to surprise it's planets with radical changes in energy output. These external factors are critical for creating a haven for life in the harsh environment of space. Our celestial neighborhood is a good one, but what about our home planet itself?
The tilt of Earth's axis controls the seasons and serves to distribute energy globally on an annual cycle. Too little tilt and seasonal changes would be insignificant and polar regions even more extreme. Too much tilt and large portions of the northern and southern hemispheres would experience long periods without sun. On a shorter time scale, Earth's 24 hour rotational period ensures daily distribution of energy to support life globally, and is a vital part of the diurnal biological clock that runs in many living organisms. Life has also evolved to take advantage of the daily rhythm of the tides, as governed by our orbiting lunar companion.
Solar System Calculators (more info)
The Animated Virtual Planetarium (more info)
Build Your Own Solar System (more info)
Earth's magnetic field shields the surface from harmful high energy radiation emitted by the sun, contributing to a natural environment that is hospitable for life. And Earth's gravity is in the range where a significant atmosphere can remain on the surface without dissipating into the vacuum of space. All of these factors are vital characteristics for a planet to support life as we know it.
Learning about tides (link to external site) (more info)
The Seasons and Earth's Orbit (link to external site) (more info)
Sun Path Activity (more info)
Solar Physics: The Sun Spot Cycle (more info)
Earth's immediate neighbors, Luna (our moon), Venus and Mars offer wonderful examples of different celestial circumstances, each with a negative outcome for sustaining life.
l to r: Venus, Earth, Luna and Mars to scale
Images courtesy NASA
Comparing the Inner Planets
Earth's moon shares nearly the same favored solar neighborhood as Earth, but lacks the mass and associated gravity to hold an atmosphere. In addition, the Moon lacks a planetary scale magnetic field to shelter its surface from the bombardment of high energy solar radiation. Without an atmosphere to distribute heat, the lunar surface fluctuates between extremely hot and cold with the monthly rising and setting of the sun. If there is any water on the moon, it is frozen beneath the surface in the perpetual shadow found at the bottom of deep polar craters.
Apollo 11 Tranquility Base
Image courtesy NASA
Venus is very near to Earth in its size and gravity, but is burdened by a dense carbon dioxide atmosphere that crushes the surface with a pressure of 90 atmospheres and drives a runaway greenhouse effect that elevates the surface temperature to over 700K, far beyond what solar proximity alone demands. The thick atmosphere may have slowed the rotation of the planet to its meager rate of once per 243 Earth days. Without a biosphere or liquid water to trap the excess carbon dioxide as carbonate rock, Venus is in a permanent heat trap, and must wait a few billion years for the sun to cool before the surface approaches conditions favorable for life (as we know it!).
Russian Venera 9 lander view of the surface of Venus
NASA rover Spirit panorama of Martian surface
Images courtesy NASA'
Mars offers the most intriguing comparison with Earth. Though only half the diameter and twice the distance from the sun as Earth, Mars supports a thin atmosphere of mostly carbon dioxide, with temperatures that may occasionally be warm enough to melt water (which would instantly boil away to vapor in the extremely low atmospheric pressure). Martian seasons and the length of day are very near to Earth's, producing polar caps that shrink and grow, dust storms, frost patches and seasonal clouds that mimic Earth. There is every indication that the surface of Mars supported vast oceans of liquid water at some point in its distant past, when it still had an atmosphere dense enought to trap solar energy. Stream channels (link to external image) (more info) , flood plains and indications of ancient shorelines sprawl in desiccated testimony to the past, and fuel the hopethat life may once have existed there during warmer times. Huge amounts of water may yet remain on Mars, frozen as permafrost beneath the surface. Mars beckons, and the 21st century opens with a fleet of spacecraft missions (link to external site) (more info) poised to unlock the mysteries of the planet's history.
Comparing the Surfaces of Earth and Mars (more info)
Returning to Earth, we are reminded once again of the cosmic confluence of factors that contribute to our currently comfortable planetary situation. But indeed, if it were not so, we would not be here to reflect upon or examine the details of the Earth system ...
For additional resources see:
Welcome to the Planets (more info)
NASA's Planetary Photojournal (more info)
Solar System Exploration (more info)